“The Secular Trend of Intelligence Test Scores: The Danish Experience for Young Men Born 1940–60200024ya”, 2021-12-09 ():
The present study investigated the Danish secular trend of intelligence test scores among young men born 1940–60200024ya, as well as the possible associations of birth cohort changes in family size, nutrition, education, and intelligence test score variability with the increasing secular trend.
The study population included all men born 1940–60200024ya who appeared before a draft board before 2020 (n = 1,556,770). At the mandatory draft board examination, the ~19-year-old men underwent a medical examination and an intelligence test. In the statistical analyses, the IQ mean and standard deviation (SD) were estimated separately for each of the included annual birth cohorts based on information from birth cohorts with available total intelligence test scores for all tested individuals (ie. 1940–18195866ya and 1987–13200024ya; the mean and SD were interpolated for the intermediate birth cohorts). Moreover, the possible associations with birth cohort changes in family size, height as a proxy for nutritional status, education, and IQ variability were investigated among those birth cohorts for whom a secular increase in intelligence test scores was found.
The results showed that the estimated mean IQ score increased from a baseline set to 100 (SD: 15) among individuals born in 1940 to 108.9 (SD: 12.2) among individuals born in 1980, since when it has decreased. Focusing on the birth cohorts of 1940–40198044ya, for whom a secular increase in intelligence test scores was found, birth cohort changes in family size, height, and education explained large proportions of the birth cohort variance in mean intelligence test scores, suggesting that these factors may be important contributors to the observed Flynn effect in Denmark.
…More specifically, we estimated an average increase of 3.8 IQ points for the birth cohorts of 1940–49, 2.5 IQ points for the birth cohorts of 1950–59, 1.3 IQ points for the birth cohorts of 1960–69, and 0.3 IQ points for the birth cohorts of 1970–79. Hereafter, we estimated an average decrease of 0.6 IQ points for the birth cohorts of 1980–89 and 1.3 IQ points for the birth cohorts of 1990–99. However, looking at the empirical data, it is clear that the estimated change for the last birth decade conceals a true decline of 0.4 IQ points from those born in 1990–199133ya, a sudden drop of 1.0 IQ points from those born in 1991–199232ya (coinciding with the change in the format of the intelligence test from a paper-and-pencil booklet to a computer-administered format), and a following stagnation. Yet the interpolated mean intelligence test scores across birth cohorts looked exactly the same irrespective of whether individuals born after 1992 were excluded from the linear regression model or not. The standard deviation declined steadily during the study period from 15.0 for individuals born in 1940 to 10.0 for individuals born in 2000. Furthermore, the symmetrical IQ distribution observed at the beginning of the study period turned into a left-skewed one concurrently with the increase in mean intelligence test scores.